For many years, manufacturers of electronic systems have recommended that users take measures to isolate their hardware from transient overvoltages (also called "surges") that may cause damage to sensitive electronic devices. Transient voltage protection systems (so-called "surge suppressors") are designed to reduce transient voltages to levels below hardware-damage susceptibility thresholds; providing such protection can be achieved through the use of various types of transient-suppressing elements coupled between the phase, neutral and/or ground conductors of an electrical distribution system.
Conventional transient-suppressing elements typically assume a high impedance state under normal operating voltages. When the voltage across a transient-suppressing element exceeds a pre-determined threshold rating, however, the impedance of the element drops dramatically, essentially short-circuiting the electrical conductors and "shunting" the current associated with the transient voltage through the element and thus away from the sensitive electronic hardware to be protected.
To be reliable, a transient-suppressing element itself must be capable of handling many typical transient-voltage disturbances without internal degradation. This requirement dictates the use of heavy-duty components designed for the particular transient voltage environment in which such elements are to be used. In environments characterized by high-magnitude or frequently-occurring transients, however, multiple transient-suppressing elements may be required.
In many applications, the transient-suppressing elements typically employed are metal-oxide varistors ("MOVs"). When designing a system incorporating MOVs it is important to recognize the limitations of such devices, and the effects that the failure of any given MOV may have on the integrity of the total system. All MOV components have a maximum transient current rating; if the rating is exceeded, the MOV may fail. An MOV component may also fail if subjected to repeated operation, even if the maximum transient current rating is never exceeded. The number of repeated operations necessary to cause failure is a function of the magnitude of transient current conducted by an MOV during each operation: the lower the magnitude, the greater the number of operations necessary to cause failure. A designer of transient voltage protection systems must consider these electrical environment factors when selecting the number and type of MOVs to be used in a particular system. Therefore, to design a reliable transient voltage suppression system, a designer must consider both the maximum single-pulse transient current to which the system may be subjected, as well as the possible frequency of transients having lower-level current characteristics.
Although individual MOVs have a maximum transient current rating, it is possible to construct a device using multiple MOVs, in parallel combination, such that the MOVs share the total transient current. In this manner, each individual MOV must only conduct a fraction of the total transient current, thereby reducing the probability that any individual MOV will exceed its rated maximum transient current capacity. Furthermore, by using a plurality of individual MOVs, a transient voltage protection system can withstand a greater number of operations because of the lower magnitude of transient current conducted by each individual MOV.
When a transient voltage suppression system incorporates multiple MOVS, it is important that the system be designed such that the failure of an individual MOV does not cause a complete loss of system functionality. When an MOV fails, due to either exceeding its maximum transient current rating or frequent operation, it initially falls into a low impedance state, drawing a large steady-state current from the electrical distribution system. This current, if not interrupted, will quickly drive an MOV into thermal runaway, typically resulting in an explosive failure of the MOV.
To avoid the explosive failure of MOVs, an appropriately-rated current-limiting element, such as a fuse, should be employed in series with MOVs. If the transient-suppressing device incorporates a plurality of parallel-coupled MOVs, however, a single fuse in series with the parallel combination of MOVs may open-circuit even if only a single MOV fails, resulting in a disconnection of the remaining functional MOVs from the electrical distribution system. Therefore, better-designed systems incorporate individual fuses for each MOV, such that the failure of an individual MOV will result only in the opening of the fuse coupled in series with the failed MOV; the remaining functional MOVs remain connected to the electrical distribution system, via their own fuses, to provide continued transient voltage protection.
In the prior art, there are transient suppression circuits that incorporate a plurality of parallel-coupled MOVs with an individual fuse provided for overcurrent protection of the MOVs. U.S. Pat. No. 5,153,806 to Corey teaches the use of a single fuse to protect a plurality of MOVs, as well as an alarm circuit for indicating when the fuse has open-circuited. Similarly, U.S. Pat. No. 4,271,466 to Comstock teaches the use of a single fuse in series with a plurality of MOVs, as well as a light-emitting diode ("LED"), coupled in parallel with the fuse, to emit light when the fuse is blown. The deficiencies of these types of circuits is that the failure of a single MOV can cause the fuse to fail whereby the remaining functional MOVs are decoupled from the circuit; i.e., the remaining functional MOVs are disconnected from the electrical distribution system and thus cannot provide continued protection from transient voltages.
There are also a limited number of transient suppression devices that employ multiple over-current limiting elements with multiple parallel-coupled MOVs or other transient suppression devices. Such devices known in the prior art, however, typically employ a bare fusible element mounted on the printed circuit board on which the MOVs are mounted. When an MOV associated with a particular fusible element fails, the fusible element typically open circuits. The open-circuiting of a fusible element is often accompanied by electrical arcing, which is particularly true in the area of transient suppression devices because of the large voltages and currents usually present when a suppression device fails. Because of the close proximity of the bare fusible elements, the electrical arcing of one fusible element can result in the destruction of adjacent elements, thereby decoupling remaining functional MOVs from the circuit and further limiting the remaining suppression capacity of the device.
The inadequacy of the prior art is that the failure of a single MOV component may cause a fuse in series with a plurality of parallel-coupled MOVs to open-circuit, thus eliminating all transient voltage suppression capability of the parallel-coupled MOVs. In prior art circuits that have employed multiple bare fusible elements with multiple parallel-coupled MOVs (or other transient suppression devices), the design of such circuits is such that the failure of a bare fusible element causes electrical arcing that can result in the destruction of adjacent fusible elements, thus resulting in further degradation of the suppression capacity of the circuit.
Therefore, what is needed in the art are apparatus for providing over-current protection to a plurality of parallel-coupled transient-suppression devices; the apparatus should provide independent over-current protection to each transient-suppression device in a combination of parallel-coupled transient-suppression devices; the apparatus should also ensure that each over-current protection element is sufficiently isolated from any arcing that may be associated with the failure of another over-current protection element in the apparatus.